rtc_irq_set_freq() requires power-of-two and associated kerneldoc

rtc_irq_set_freq() requires power-of-two and associated kerneldoc

RTC periodic IRQs are only defined to work for 2^N Hz values.  This patch
moves that validity check into the infrastructure, so drivers don't need to
check it; and adds kerneldoc for the two interface functions related to
periodic IRQs.  (One of which was quite mysterious until its first use was
recently checked in!)

Signed-off-by: David Brownell <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>